<!DOCTYPE html>
<html>
<head>
	<meta charset="utf-8">
	<meta name="viewport" content="width=device-width, initial-scale=1">
	<title></title>
	<style type="text/css">
		*{
			margin: 0;
			padding: 0;
		}
		.nav{
			width: 100%;
			height: 50px;
			background: black;
			line-height: 50px;
		}
		.first{
			color: #fff;
			list-style: none;
			float: left;
			padding: 0 40px;
			background: lightpink;
			margin: 0 20px;
			position: relative;
		}
		.erji li{
			list-style: none;
		}
		.erji{
			width: 100%;
			position:absolute;
			bottom: -150;
			left: 0;
			background: #ccc;
			text-align: center;
			display: none;
		}
		.first:hover .erji{
			display: block;
		}

	</style>
</head>
<body>
	<div class="nav">
		<ul>
			<li class="first">首页
 				<div class="erji">
 					<ul>
 						<li>二级导航</li>
 						<li>二级导航</li>
 						<li>二级导航</li>

 					</ul>
 				</div>
			</li>
			<li class="first">首页</li>
			<li class="first">首页</li>
			<li class="first">首页</li>

		</ul>
	</div>
</body>
</html>